In the field of power electronics, transistors for a multiplicity of applications have to be able to carry current in the reverse direction. In the case of the MOSFET, the intrinsic diode thereof is used for that purpose. The standard IGBT, on account of its internal structure, is not able to conduct current in the reverse direction without a further component. If reverse conductivity is required, a diode is usually connected in antiparallel with the IGBT component. However, this results in some undesired consequences. The housing for at least one IGBT has to be selected with an appropriate size, such that there is space not only for the transistor chip but also for the diode chips. The wiring outlay during bonding increases since now a higher number of chips have to be interconnected. The entire manufacturing chain necessitates implementing greater outlay and effort since special diode chips also have to be manufactured, measured, kept available and processed, besides the transistor chips. By decoupling the forward (IGBT) and reverse current carrying (diode), both processes are thermally decoupled to the greatest possible extent. The thermal resistance for the individual elements is correspondingly high.
It is known for an IGBT and a diode to be monolithically integrated in one another. This is done by the p-doped collector zone (p-type emitter) of the IGBT being locally interrupted. At these locations, n-doped semiconductor material (n-type cathode of the diode) contacts the collector metalization. A PIN diode structure thus arises between the emitter, the lightly doped drift zone and the p-doped material in the MOS channel region. Such an IGBT is referred to as a reverse-conducting IGBT (for short: RC-IGBT). Such an RC-IGBT is described for example in the document US 2007/0231973 A1 (Rüthing, Schulze, Niedernostheide, Hille).
While the conducting behavior of a separate diode connected in antiparallel with the IGBT exhibits no dependence on the drive state of the IGBT, the situation is different for the RC-IGBT depending on the design of the component. If the RC-IGBT is driven in reverse-conducting operation (i.e. in diode operation), then the MOS channel of the IGBT is conducting. Since the latter can carry current in both directions, an additional current path, in parallel with the internal diode, arises for the electrons of the reverse current. Since this means that now not all the electrons contribute to the flooding of the PIN diode structure, the forward voltage drop thereof can increase significantly, which is generally an undesired effect. For this reason, in most applications it is desirable to drive the gate of a reverse-conducting IGBT in the off state in diode operation (i.e. to turn off the MOS channel of the IGBT).
Upon transition from the on state to the off state, the charge stored in the component in the steady-state diode flooding situation should be extracted in order to keep the dynamic losses small. This quantity of charge is also called “reverse recovery” charge and has a considerable influence on the diode switch-off losses and on the IGBT switch-on losses of the commutation partner (for example a further IGBT in the case of an arrangement of two IGBTs in a half-bridge). On account of the properties described, low-loss diode operation can be implemented if, in the operating sequence of the application circuit, different states of the intrinsic freewheeling diode with regard to the flooding thereof (charge carrier flooding) are combined in such a way that the sum of application-specifically arising static and dynamic losses yields a minimum.